Haemostasis is initiated by the formation of a complex between tissue factor (TF) being exposed to the circulating blood following an injury to the vessel wall, and FVIIa which is present in the circulation in an amount corresponding to about 1% of the total FVII protein mass. This complex is anchored to the TF-bearing cell and activates FX into FXa and FIX into FIXa on the cell surface. FXa activates prothrombin to thrombin, which activates FVIII, FV, FXI and FXIII. Furthermore, the limited amount of thrombin formed in this initial step of haemostasis also activates the platelets. Following the action of thrombin on the platelets these change shape and expose charged phospholipids on their surface. This activated platelet surface forms the template for the further FX activation and the full thrombin generation. The further FX activation on the activated platelet surface occurs via a FIXa–FVIIIa complex formed on the surface of the activated platelet, and FXa then converts prothrombin into thrombin while still on the surface. Thrombin then converts fibrinogen into fibrin which is insoluble and which stabilises the initial platelet plug. This process is localized to the site of TF exposure thereby minimizing the risk of a systemic activation of the coagulation system.
FVIIa exists in plasma mainly as a single-chain zymogen, which is cleaved by FXa into its two-chain, activated form, FVIIa. Recombinant activated factor VIIa (rFVIIa) has been developed as a pro-haemostatic agent. The administration of rFVIIa offers a rapid and highly effective pro-haemostatic response in haemophilic subjects with bleedings who cannot be treated with coagulation factor products due to antibody formation. Also bleeding subjects with a factor VII deficiency can be treated successfully with FVIIa. In these studies, no unfavourable side effects of rFVIIa (in particular the occurrence of thromboembolism) has been encountered.
Extra exogenously administered FVIIa increases the formation of thrombin on the activated platelet surface. This occurs in haemophilia subjects lacking FIX or FVIII and therefore missing the most potent pathway for full thrombin formation. Also in the presence of a lowered number of platelets or platelets with a defect function, extra FVIIa increases the thrombin formation and is haemostatically effective in subjects.
The coagulation factors of the extrinsic pathway are inhibited by EPI (Extrinsic Pathway Inhibitor) now called TFPI (Tissue Factor Pathway Inhibitor). TFPI is a Kunitz type protease inhibitor which binds and inhibits FXa. The TFPI-FXa complex inhibits FVIIa-TF (Rapaport, Blood 73, p 359, 1989). The significance of TFPI for the coagulation reaction has not yet been established since only unphysiological high concentrations affects conventional coagulation assays (Broze et al., Bio-chemistry 29, p 7539, 1990). Therefore, TFPI activity is measured in sophisticated assays where TFPI is allowed to bind FXa and the TFPI-FXa is added to TF-FVII reactants that may activate FIX or FX (Bajaj et al., J Clin Invest 79, p 1974, 1987; Sandset et al., Thromb Res 47, p 389, 1987).
It is well known that subjects who bleed excessively in association with surgery or major trauma and need blood transfusions develop more complications than those who do not experience any bleeding. However, also moderate bleedings requiring the administration of human blood or blood products (platelets, leukocytes, plasma-derived concentrates for the treatment of coagulation defects, etc.) may lead to complications associated with the risk of transferring human viruses (hepatitis, HIV, parvovirus, and other, by now unknown viruses). Extensive bleedings requiring massive blood transfusions may lead to the development of multiple organ failure including impaired lung and kidney function. Once a subject has developed these serious complications a cascade of events involving a number of cytokines and inflammatory reactions is started making any treatment extremely difficult and unfortunately often unsuccessful. Therefore a major goal in surgery as well as in the treatment of major tissue damage is to avoid or minimise the bleeding
To avoid or minimise such bleeding it is of importance to ensure the formation of stable and solid haemostatic plugs that are not easily dissolved by fibrinolytic enzymes. Furthermore, it is of importance to ensure quick formation of such plugs or clots.
European Patent No. 225.160 (Novo Nordisk) concerns compositions of FVIIa and methods for the treatment of bleeding disorders not caused by clotting factor defects or clotting factor inhibitors.
European Patent No. 82.182 (Baxter Travenol Lab.) concerns a composition of factor VIIa for use in counteracting deficiencies of blood clotting factors or the effects of inhibitors to blood clotting factors in a subject.
International Patent Publication No. WO 93/06855 (Novo Nordisk) concerns the topical application of FVIIa.
European Patent No. 558529 (Novo Nordisk) concerns compositions for the treatment of patients with prolonged coagulation time, the composition containing, as an active component, an EPI inhibitor.
International Patent Publication No. WO 96/28153 (Novo Nordisk) and U.S. Pat. No. 5,622,988 (Novo Nordisk) concerns novel compounds which show activity in inhibiting Tissue Factor Pathway Inhibitor (TFPI), and therefore can be used in reducing coagulation time in medical conditions which are characterized by excess or uncontrolled bleeding.
There remains a need in the art for an improved, reliable and widely applicable method of enhancing thrombin generation, forming quick stable haemostatic plugs and achieving full haemostasis in subjects, in particular in subjects having an impaired thrombin generation. There is also a need for a method wherein the amount of FVIIa needed for achieving full haemostasis is lowered.